Intense interest has developed related to the direct electrochemical detection of aliphatic compounds based on electrocatalytic reactions at noble metal electrodes, chiefly Au and Pt. Electrochemical detection is a widely accepted means of detection in liquid and ion chromatography. Electrochemical detectors operate by applying an electric potential to the working electrode in a flow-through cell. Such detectors typically employ a three-electrode cell including a working electrode, a reference electrode and a counter electrode. The methodology relies on use of multi-step potential waveforms which incorporate a detection operation along with the anodic cleaning and cathodic reactivation of the electrode surface. A typical potential waveform is shown in FIG. 1A. Anodic detection occurs at potential E.sub.1 with current sampling during a 16.7-ms period at the end of period t.sub.1. The potential then is stepped to E.sub.2 (period t.sub.2), for oxidative cleaning of the electrode surface, and subsequently to E.sub.3 (period t.sub.3) for reactivation by cathodic dissolution of the surface oxide formed at E.sub.1 and/or E.sub.2. Adsorption of analyte also can occur at E.sub.3 and for long t.sub.3 the concentration of analyte at the electrode surface is reestablished to the value of the bulk solution (C.sup.s =C.sup.b). The analytical application of this method, now known as Pulsed Amperometric Detection (PAD), has been demonstrated for alcohols, polyalcohols and carbohydrates (reducing and non-reducing) (1-6); amines and amino acids (primary and secondary) (7); aminoglycosides (8); and numerous sulfur compounds (except sulfate, sulfonic acids and sulfones) (9-11).
Recently, Pulsed Coulometric Detection (PCD) was described (12). The significant difference between PAD and PCD lies in the instrumental protocol related to measurement of the faradaic signal. In PAD, electrode current is averaged over a time period of 16.7 ms (i.e., 1/60 Hz.sup.-1) whereas in PCD the amperometric response is electronically integrated over an integral number of sequential 16.7-ms periods (12). PCD inherently has a larger signal-to-noise ratio (S/N) because of the larger signal strength and because the integral of a 60-Hz correlated noise signal, a predominant form of noise in electronic instrumentation, remains at zero over the integration period.
With the advent of more complex liquid chromatographic techniques which employ a variety of gradient elution methods, it is necessary to develop detectors which are capable of sustaining high values of sensitivity and detectability over the gradient period, and which also can reject automatically the accompanying variation in background signal. Detectors traditionally used for aliphatic compounds cannot be used in pH-gradient liquid chromatography (LC) because they are affected by changes in ionic strength. Photometric detection suffers because of an inherently low sensitivity for aliphatic compounds without extensive .pi.-bonding and because of baseline drift which accompanies a change in the refractive index of the mobile phase. Refractive index detection is strongly affected by concentration gradients and the baseline shift observed for even small changes in mobile phase composition can overwhelm the analyte signal.
The methods of PAD (1-10) and PCD (12) were introduced for detection of numerous aliphatic organic compounds in conjunction with liquid chromatography (LC). However, the ability of either technique to resist even a slight pH change is strongly dependent on the detection potential and the electrode material selected. In fact, PAD at a Pt electrode in a flow-injection (FI) system has been suggested for the determination of pH changes in caustic media (13). Sensitivity of the baseline in PAD and PCD to changes in solution pH is greatest for amines and sulfur compounds at Au and Pt because the anodic detection reactions are catalyzed by simultaneous formation of surface oxide on the noble metal electrodes (7,9,10).